fix(relay): fix llm_routing read path bug and add User-Agent header for Coding Plan

1. connectionStore.ts: storedAccount.account.llm_routing → storedAccount.llm_routing
   - saveSaaSSession stores SaaSAccountInfo directly, not { account: SaaSAccountInfo }
   - This bug caused admin llm_routing config to never take effect

2. relay/service.rs: add User-Agent: claude-code/1.0 header
   - Kimi Coding Plan requires recognized coding agent User-Agent
   - Default reqwest UA is rejected with 403

3. Docs: add llm_routing routing mode explanation and troubleshooting entries

desktop/src/store/connectionStore.ts

@@ -358,8 +358,9 @@ export const useConnectionStore = create<ConnectionStore>((set, get) => {
           const raw = localStorage.getItem('zclaw-saas-account');
           if (raw) {
             const storedAccount = JSON.parse(raw);
+            // storedAccount is SaaSAccountInfo (saved directly by saveSaaSSession)
             // 类型安全解析: 仅接受 'relay' | 'local' 两个合法值
-            const adminRouting = storedAccount?.account?.llm_routing;
+            const adminRouting = storedAccount?.llm_routing;
             if (adminRouting === 'relay') {
               // Force SaaS Relay mode — admin override
               localStorage.setItem('zclaw-connection-mode', 'saas');